The protocol uses Core Lab resources only. The broad objective of this research proposal is to gain knowledge about the molecular mechanisms of thyroid physiology through the identification and characterization of human genetic errors causing defects of thyroid hormone regulation, synthesis, transport and action. Central to this project is the availability of patient material for clinical studies. These are: subjects expressing the phenotype of thyrotropin resistance, families with iodide trapping defect, iodide organification defect, defects of thyroxine-binding globulin, transthyretin and albumin, and resistance to thyroid hormone.